This invention relates to a method for working the surface of electrical sheet or strip products to affect the domain size so as to reduce the core loss properties. More particularly, this invention relates to providing localized strains in the surface of electrical steels by electron beam treatment to provide heat resistant domain refinement.
In the manufacture of grain oriented silicon steel, it is known that the Goss secondary recrystallization texture, (110) [001] in terms of Miller's indices, results in improved magnetic properties, particularly permeability and core loss over nonoriented silicon steels. The Goss texture refers to the body-centered cubic lattice comprising the grain or crystal being oriented in the cube-on-edge position. The texture or grain orientation of this type has a cube edge parallel to the rolling direction and in the plane of rolling, with the (110) plane being in the sheet plane. As is well known, steels having this orientation are characterized by a relatively high permeability in the rolling direction and a relatively low permeability in a direction at right angles thereto.
In the manufacture of grain-oriented silicon steel, typical steps include providing a melt having on the order of 2-4.5% silicon, casting the melt, hot rolling, cold rolling the steel to final gauge, typically 7 or 9 mils, and up to 14 mils, with an intermediate annealing when two or more cold rollings are used, decarburizing the steel, applying a refractory oxide base coating, such as a magnesium oxide coating, to the steel, and final texture annealing the steel at elevated temperatures in order to produce the desired secondary recrystallization and purification treatment to remove impurities such as nitrogen and sulfur. The development of the cube-on-edge orientation is dependent upon the mechanism of secondary recrystallization wherein during recrystallization, secondary cube-on-edge oriented grains are preferentially grown at the expense of primary grains having a different and undesirable orientation.
Grain-oriented silicon steel is conventionally used in electrical applications, such as power transformers, distribution transformers, generators, and the like. The domain structure and resistivity of the steel in electrical applications permits cyclic variation of the applied magnetic field with limited energy loss, which is termed "core loss". It is desirable, therefore, in steels used for such applications, that such steels have reduced core loss values.
As used herein, "sheet" and "strip" are used interchangeably and mean the same unless otherwise specified.
It is also known that through the efforts of many prior art workers, cube-on-edge grain-oriented silicon steels generally fall into two basic categories: first, regular or conventional grain oriented silicon steel and second, high permeability grain oriented silicon steel. Regular grain oriented silicon steel is generally characterized by permeabilities of less than 1850 at 10 Oersteds with a core loss of greater than 0.400 watts per pound (WPP) at 1.5 Tesla at 60 Hertz for nominally 9 mil material. High permeability grain oriented silicon steels are characterized by higher permeabilities and lower core losses. Such higher permeability steels may be the result of compositional changes alone or together with process changes. For example, high permeability silicon steels may contain nitrides, sulfides and/or borides which contribute to the precipitates and inclusions of the inhibition system which contribute to the properties of the final steel product. Furthermore, such high permeability silicon steels generally undergo cold reduction operations to final gauge wherein a final heavy cold reduction on the order of greater than 80% is made in order to facilitate the grain orientation.
It is known that domain size and thereby core loss values of electrical steels, such as amorphous materials and particularly grain-oriented silicon steels, may be reduced if the steel is subjected to any of various practices to induce localized strains in the surface of the steel. Such practices may be generally referred to as "scribing" or "domain refining" and are performed after the final high temperature annealing operation. If the steel is scribed after the final texture annealing, then there is induced a localized stress state in the texture annealed sheet so that the domain wall spacing is reduced. These disturbances typically are relatively narrow, straight lines, or scribes generally spaced at regular intervals. The scribe lines are substantially transverse to the rolling direction and typically are applied to only one side of the steel.
In the use of such amorphous and grain-oriented silicon steels, the particular end use and the fabrication techniques may require that the scribed steel product survive a stress relief anneal (SRA), while other products do not undergo such an SRA. During fabrication incident to the production of stacked core transformers and, more particularly, in the power transformers of the United States, there is a demand for a flat, domain refined silicon steel which is not subjected to stress relief annealing. In other words, the scribed steel does not have to provide heat resistant domain refinement.
During the fabrication incident to the production of other transformers, such as most distribution transformers in the United States, the steel is cut and subjected to various bending and shaping operations which produce stresses in the steel. In such instances, it is necessary and conventional for manufacturers to stress relief anneal the product to relieve such stresses. During stress relief annealing, it has been found that the beneficial effect on core loss resulting from some scribing techniques, such as thermal scribing, are lost. For such end uses, it is required and desired that the product exhibit heat resistant domain refinement (HRDR) in order to retain the improvements in core loss values resulting from scribing.
It has also been suggested in prior patent art that electron beam technology may be suitable for scribing silicon steel. U.S. Pat. No. 3,990,923-Takashina et al., dated Nov. 9, 1976 discloses that electron beams may be used on primary recrystallized silicon steel to control or inhibit the growth of secondary recrystallization grains. U.S. Pat. No. 4,554,029-Schoen et al., dated Nov. 19, 1985, generally discloses that electron beam resistance heating may be used on finally annealed electrical steel if damage of the insulated coating is not of concern. The damage to the insulative coating and requirements of a vacuum were considered to be major drawbacks. There is no teaching or suggestion in the art, however, of any actual or practical use of electron beam technology for scribing electrical steels.
A copending application, Ser. No. 163,448, filed Mar. 3, 1988, by the assignee of this invention discloses a method and apparatus of electron beam treatment to affect domain size and improve core loss without damaging any coating thereon or changing the shape thereof.
What is needed is a method and apparatus for treating electrical sheet products to effect domain refinement which is heat resistant and can withstand a stress relief anneal (SRA) typically used in the fabrication of transformers. Still further, the method and apparatus should be suitable for treating grain-oriented silicon steels of both the high permeability and conventional types as well as amorphous type electrical materials.